The present invention pertains to improvements to thermal components of a Stirling cycle heat engine and more particularly to the heater head and combustion chamber assembly and regenerator.
Stirling cycle machines, including engines and refrigerators, have a long technological heritage, described in detail in Walker, Stirling Engines, Oxford University Press (1980), incorporated herein by reference. The principle underlying the Stirling cycle engine is the mechanical realization of the Stirling thermodynamic cycle: isovolumetric heating of a gas within a cylinder, isothermal expansion of the gas (during which work is performed by driving a piston), isovolumetric cooling, and isothermal compression.
Additional background regarding aspects of Stirling cycle machines and improvements thereto are discussed in Hargreaves, The Phillips Stirling Engine (Elsevier, Amsterdam, 1991) and in co-pending U.S. patent applications Ser. No. 09/115,383, filed Jul. 14, 1998, and Ser. No. 09/115,381, filed Jul. 14, 1998, which reference and both of which applications are herein incorporated by reference.
The principle of operation of a Stirling engine is readily described with reference to FIGS. 1a-1e, wherein identical numerals are used to identify the same or similar parts. Many mechanical layouts of Stirling cycle machines are known in the art, and the particular Stirling engine designated generally by numeral 10 is shown merely for illustrative purposes. In FIGS. 1a to 1d, piston 12 and a displacer 14 move in phased reciprocating motion within cylinders 16 which, in some embodiments of the Stirling engine, may be a single cylinder. A working fluid contained within cylinders 16 is constrained by seals from escaping around piston 12 and displacer 14. The working fluid is chosen for its thermodynamic properties, as discussed in the description below, and is typically helium at a pressure of several atmospheres. The position of displacer 14 governs whether the working fluid is in contact with hot interface 18 or cold interface 20, corresponding, respectively, to the interfaces at which heat is supplied to and extracted from the working fluid. The supply and extraction of heat is discussed in further detail below. The volume of working fluid governed by the position of the piston 12 is referred to as compression space 22.
During the first phase of the engine cycle, the starting condition of which is depicted in FIG. 1a, piston 12 compresses the fluid in compression space 22. The compression occurs at a substantially constant temperature because heat is extracted from the fluid to the ambient environment. The condition of engine 10 after compression is depicted in FIG. 1b. During the second phase of the cycle, displacer 14 moves in the direction of cold interface 20, with the working fluid displaced from the region of cold interface 20 to the region of hot interface 18. This phase may be referred to as the transfer phase. At the end of the transfer phase, the fluid is at a higher pressure since the working fluid has been heated at constant volume. The increased pressure is depicted symbolically in FIG. 1c by the reading of pressure gauge 24.
During the third phase (the expansion stroke) of the engine cycle, the volume of compression space 22 increases as heat is drawn in from outside engine 10, thereby converting heat to work. In practice, heat is provided to the fluid by means of a heater head 100 (shown in FIG. 2) which is discussed in greater detail in the description below. At the end of the expansion phase, compression space 22 is full of cold fluid, as depicted in FIG. 1d. During the fourth phase of the engine cycle, fluid is transferred from the region of hot interface 18 to the region of cold interface 20 by motion of displacer 14 in the opposing sense. At the end of this second transfer phase, the fluid fills compression space 22 and cold interface 20, as depicted in FIG. 1a, and is ready for a repetition of the compression phase. The Stirling cycle is depicted in a P-V (pressure-volume) diagram as shown in FIG. 1e. 
Additionally, on passing from the region of hot interface 18 to the region of cold interface 20, the fluid may pass through a regenerator 134 (shown in FIG. 2). Regenerator 134 is a matrix of material having a large ratio of surface area to volume which serves to absorb heat from the fluid when it enters hot from the region of hot interface 18 and to heat the fluid when it passes from the region of cold interface 20.
Stirling cycle engines have not generally been used in practical applications due to several daunting engineering challenges to their development. These involve such practical considerations as efficiency, lifetime, and cost. The instant invention addresses these considerations.
In accordance with preferred embodiments of the present invention, there is provided a thermal cycle engine of the type having a piston undergoing reciprocating linear motion within an expansion cylinder containing a working fluid heated by heat from an external source that is conducted through a heater head. The thermal cycle engine has a heat exchanger for transferring thermal energy across the heater head from a heated external fluid to the working fluid, the heat exchanger comprising a set of heat transfer pins, wherein each heat transfer pin has an axis directed away from the cylindrical wall of the expansion cylinder. In accordance with alternate embodiments of the invention, the axis of each heat transfer pin may be substantially perpendicular to the cylindrical wall of the expansion cylinder. In accordance with further alternate embodiments of the invention, the heat exchanger may comprise a set of fins substantially aligned with the axis of the expansion cylinder. The thermal cycle engine may further include a plurality of dividing structures for spatially separating the set of heat transfer pins into subsets of heat transfer pins, and the heat transfer pins of each subset of heat transfer pins may have axes that are substantially parallel to each other.
In accordance with other embodiments of the invention, a subset of the set of heat transfer pins, up to the entirety thereof, may include heat transfer pins extending from the heater head into the external fluid. A pin backer may be provided for guiding the heated external fluid past the set of heat transfer pins. A dimension of the pin backer perpendicular to the heater head may decrease in the direction of the flow path, and the surface area of the heat transfer pins transverse to the flow path may increase in the direction of the flow path. The heat transfer pins may have a population density that increases in the direction of the flow path, and the height and density of the heat transfer pins may vary with distance in the direction of the flow path.
In accordance with another aspect of the present invention, a method is provided for manufacturing a heat exchanger for transferring thermal energy across a heater head from a heated external fluid to the working fluid. The method has the steps of casting at least one array of heat transfer pins integrally cast onto a panel, bonding the array of heat transfer pins to the heater head. The step of bonding may include mechanically attaching the panel to the heater head and may also include brazing the panel of the array of heat transfer pins to the heater head.
A method for manufacturing a heat exchanger in accordance with further embodiments of the invention provides the steps of fabricating a plurality of perforated rings, stacking the perforated rings in contact with a heater head, and bonding the perforated rings to the heater head. The step of fabricating may include stamping the rings out of a sheet of metal.
In accordance with yet a further aspect of the invention, a thermal sensor is provided for measuring a temperature of the heater head of a thermal cycle engine at a point of maximum temperature of the heater head. The thermal sensor may be a thermocouple, and a fuel regulator may regulate the fuel supply on the basis of at least the temperature of the heater head at the point of maximum temperature.
In accordance with another embodiment of the invention, a thermal cycle engine may have a secondary ring burner supplementary to the main combustor for supplying additional fuel to cause additional combustion of the exhaust gas.
In accordance with yet another embodiment of the invention, a regenerator for a thermal cycle engine is provided having a random network of fibers formed to fill a specified volume and a material for cross-linking the fibers at points of close contact between fibers of the network. The fibers may be metal, including steel wool, and the material for cross-linking the fibers may be nickel. The fibers may be silica glass and the material for cross-linking the fibers may be tetraethylorthosilicate.
A regenerator for a thermal cycle engine, in accordance with alternate embodiments of the invention, may have a volume defined by an inner sleeve and an outer sleeve, the inner and outer sleeves being substantially concentric, and two parallel planes, each substantially perpendicular to each of the inner and outer sleeves. A random network of fibers is contained within the volume and two parallel screens coupled to both the inner and outer sleeves contain the random network of fibers within the volume.
In accordance with other embodiments of the invention, a method for manufacturing a regenerator for a thermal cycle engine is provided. The method has the steps of filling a form with a random network of electrically conducting fibers, immersing the form in an electroplating solution, and applying a current between the solution and the random network of fibers in such a manner as to deposit a material for cross-linking the electrically conducting fibers at points of close contact between fibers. Alternatively, the form may be filled with a random network of fibers and the random network of fibers may be sintered in such a manner as to cross-link the fibers at points of close contact between fibers.
Yet another method is provided for manufacturing a regenerator for a thermal cycle engine, having the steps of forming a reticulated foam into a specified shape, depositing a ceramic slurry onto the reticulated foam, heat treating the slurry in such a manner as to burn off the foam, and sintering the ceramic.
A method is provided for controlling a measured temperature of a part of a heater head of a thermal cycle engine having an external combustor, the method comprising regulating a fuel flow to the external combustor. And a method is provided for distributing heat circumferentially around a heater head of a thermal cycle engine, the heater head having an interior surface, the method comprising the step of applying a layer of high-thermal-conductivity metal to at least one of the interior and exterior surfaces of the heater head.